This invention relates in general to turbocompressors and in particular to a new and useful method and apparatus for controlling the operation of the turbocompressor using a system having a blow-off control operating curve.
The flowback of pumped medium in surges from the compression side to the suction side is called pumping in compressors. This condition sets in when the end pressure or the ratio of end pressure to suction pressure is too high and/or the throughput is too low. In the characteristic field the throughput or pressure, therefore, can be evaluated to form a pumping limit line or graphic curve separating the characteristic field into a stable and instable zone. The pumping limit line is curved, i.e. it is flatter when the pressure rises.
To protect compressors against pumping, a blow-off line at a safety distance parallel to the pumping limit line is defined, and if the momentary working point approaches the blow-off line a blow-off valve is opened so that the actual value of a control variable, in particular the throughput, does not exceed a set-point value determined by way of the blow-off line and the command vaiable, in particular the end pressure. There are also controls in which the throughput is the command variable to form the set-point value while the end pressure is the variable to be controlled to the set-point value.
Due to the curving of the blow-off line, a fixed change of the command variable will give different charges of the set point, depending on the location along the blow-off line. This has the effect of producing different amplifications on the control circuit.
Pumping limit controls are safety controls and are usually activiated so that they work close to the stability limit to assure the best possible compressor protection. The location of the stability limit is influenced very much by the overall amplification of the control circuit. A high overall amplification is most likely to lead to instability. The amplification factor of the actual controller, therefore, is adjusted so that it, together with the amplification resulting from the gradient of the blow-off line, leads to an overall amplification still within the stability limit. Of course, the adjustment should affect the blow-off line section in which the highest amplification is effective. In other blow-off line sections, to which may also belong the most frequent working ranges, the control circuit is then not optimally adjusted. Therefore, it is a consequence of the severely curved blow-off line that a pumping limit controller with fixed control parameters is not optimally adjusted in wide working ranges.